Digital Filter Topology for Faster Overload Recovery
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Direct-form digital filters, particularly those using fixed-point arithmetic, experience prolonged recovery from overload conditions due to internal distortion and frequency shifts caused by clipping, leading to slow or incomplete recovery from overload.
Innovation Solution
A digital filter design that includes a first integrator stage with controlled feedback and a second stage capable of controlled clipping, ensuring that clipping occurs at the output stage, reducing distortion and enabling faster recovery by cascading multiple integrator and unit-delay pairs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If direct-form digital filters are used with fixed-point arithmetic, then implementation efficiency is improved with only N delay stages for Nth order filter, but overload recovery time increases significantly due to internal distortion and frequency shifts
Solution Approach 1:
The filter is divided into multiple second-order sections (SOS) rather than implementing a single high-order direct-form filter. Each SOS is independently structured with specific coefficient relationships that prevent internal overload while maintaining overall filter performance. This segmentation allows each section to operate within safe ranges even when the overall filter experiences overload conditions.
Solution Approach 2:
The patent transforms the filter implementation from direct-form to a cascade of second-order sections with specific parameter relationships. By changing the structural parameters and coefficient relationships (where b1 = a1 and b2 = a2 for each section), the filter achieves better numerical stability and faster recovery from overload conditions while maintaining the same transfer function.
2Object-affected harmful factors
If clipping occurs at internal summing nodes in direct-form filters, then output signal levels are limited, but internal distortion increases and causes prolonged error periods
Solution Approach 1:
The harmful clipping effect is extracted and relocated to the final output stage of each second-order section rather than occurring at internal summing nodes. By using controlled clipping at the output combiner where it least affects internal state variables, the patent separates the signal limiting function from the distortion-generating location, allowing internal nodes to maintain accurate representations.
Solution Approach 2:
The patent introduces intermediate output combiners at each second-order section that act as mediators between internal processing nodes and the next stage. These intermediaries provide controlled clipping points that protect internal nodes from overload while maintaining signal integrity through the use of feedback paths and proper coefficient relationships.
3Reliability
If clipping changes the transfer function of the filter, then corner and center frequencies move away from design values, but this structural change cannot be modified
Solution Approach 1:
Each second-order section incorporates feedback paths from the output combiner back to the input summing node with coefficients a1 and a2. This feedback mechanism compensates for the effects of clipping by continuously adjusting the internal state variables to maintain the correct transfer function and frequency characteristics, preventing frequency shifts even when clipping occurs at the output.
Data Source
AI summary
A digital filter having improved overload characteristics provides improved performance in audio equalizers and other systems. In contrast to a standard digital filter, clipping is enforced at the output of the filter and an integrator is used to implement the first filter stage, which is then followed by another stage that may be a unit delay or an integrator. Scalers and combiners are provided to scale an input signal representation and the output signal representation and combine them to provide the particular coefficient inputs to the integrator and the second stage forming a direct form filter. The resulting filter implements the same transfer function as a corresponding direct form filter, with an improved recovery from internal overload conditions. Higher-order filters can be formed by cascading the second-order filters formed by multiple integrator/second stage pairs.